1. Field of the Invention
The present invention relates to a resistive superconducting fault current limiter, and more particularly, to a resistive superconducting fault current limiter capable of reducing a current loss of a superconducting limiter element due to a self magnetic field and an external magnetic field when a normal current is applied thereto, and capable of uniformly quenching the superconducting limiter element when a fault current such as a short circuit is applied thereto.
2. Description of the Background Art
A fault current limiter in an electric power system serves to limit mechanical, thermal, and electrical stresses applied to a bus bar, an insulator, a breaker, etc. of the electric power system when a fault current such as a short circuit, a ground fault, and a lightning strike occurs.
A superconducting fault current limiter utilizes a principle that a superconductor is quenched by a fault current exceeding a threshold current, that is, a superconductor is transited to a resistive state after its superconducting characteristic is lost.
A fault current flows to the superconducting fault current limiter due to a power system fault, the superconducting fault current limiter transits to a resistor having a high impedance thereby to prevent the fault current.
Generally, a superconductor having a specific resistance of ‘0’(‘zero’) in a superconducting state is transited to a normal conducting state where the specific resistance is not ‘0’(‘zero’) due to existence of a current or a magnetic field and a temperature characteristic. The phenomenon that the superconductor loses its superconducting characteristic is called as a ‘quench’.
The superconducting fault current limiter is largely grouped as an inductive type and a resistive type. The resistive type superconducting fault current limiter has a simpler structure, a lighter weight, and a cheaper fabrication cost when compared with the inductive type one since a superconductor manages all operations including a fault current detection, a state transition to a resistor, and a current limiting.
A construction of the conventional resistive type superconducting fault current limiter will be explained with reference to FIG. 1.
The conventional resistive type superconducting fault current limiter comprises a cryostat 14 formed of a non-metallic material and filled with a refrigerant such as liquid nitrogen; a superconducting limiter element contained in the refrigerant inside the cryostat 14 and maintained as a superconductive state; a foil coil 16 installed outside the cryostat 14 so as to surround the cryostat 14, and formed of copper or aluminum, for applying a uniform magnetic field in a horizontal direction to the superconducting limiter element 12; current leads 13 and 15 for connecting the superconducting limiter element 12 and the foil coil 16 in series; and a varistor 17 connected to the series circuit between the superconducting limiter element 12 and the foil coil 16 in parallel, for preventing an over-voltage generated at the series circuit transiently.
The conventional resistive type superconducting fault current limiter is connected to a power line of an electric power system.
When a fault current flows to the resistive type superconducting fault current limiter, a uniform and strong magnetic field in a horizontal direction is applied to the superconducting limiter element 12 from the foil coil 16. Then, the superconducting limiter element 12 becomes a state exceeding a threshold current density and a threshold magnetic field density, and thus is transited to a normal conductive state having a resistance. Therefore, the fault current is limited.
In the resistive type superconducting fault current limiter, the foil coil 16 applies a strong magnetic field to the superconducting limiter element 12 in the event of a large fault current, thereby quenching the superconducting limiter element 12 uniformly and fast. However, even when a normal current less than a rated current flows the resistive type superconducting fault current limiter, the foil coil 16 applies a magnetic field to the superconducting limiter element 12 and the superconducting limiter element 12 generates a self magnetic field, which causes the superconducting limiter element 12 to have a great current loss. Therefore, in is order to increase an amount of a current flowing to the superconducting limiter element 12 by compensating the current loss, the number of the superconducting limiter elements 12 has to be increased, which causes a size and the number of the foil coil 16 to be increased. The problems become serious when a rated voltage of the electric power system is higher.
Since the current loss is represented as a heat emission, in order to maintain a temperature of 65K˜77K(Kelvin) which is a temperature to maintain the superconducting limiter element in a superconductive state, a cooling device such as a larger cryostat is required.
Since a current always flows to the foil coil 16, a thermal shielding means of the cryostat 14 for a heat emission of the heated foil coil 16 is necessary.
In the conventional superconducting fault current limiter, a magnetic field generated from the foil coil 16 is applied to the superconducting limiter element 12 in a horizontal direction thereby to cause a uniform and simultaneous quenching of the superconducting limiter element. The above method is applied to a BISCCO-based superconducting limiter element thus to obtain the same effect. However, when the above method is applied to a YBCO-based superconducting limiter element, a magnetic field necessary to apply a horizontal magnetic field has to be larger than a magnetic field necessary to apply a vertical magnetic field by several tens times. In order for the foil coil 16 to generate a large magnetic field, the size and the number of the foil coils have to be increased. As the result, the superconducting fault current limiter has an increased size thus to have a difficulty in obtaining an installation space thereof, and a fabrication cost thereof is increased